1. Field of the Invention
The present invention relates to a superconductor filter for transmitting-receiving signals used in a radio transmitter-receiver apparatus and a radio transmitter-receiver apparatus using a superconductor filter.
2. Description of the Related Art
In, for example, a radio transmitter-receiver apparatus used in a base station for mobile communication, a receiver filter and a transmitter filter are housed as constituents important for selecting a desired frequency band alone. In general, transmitter signals are generated by dividing into at least two channels and are finally synthesized by using a synthesizer so as to be sent from an antenna. In recent years, a frequency around 2 GHz is used in a mobile communication that is rapidly propagated. However, the frequency band assigned to each carrier is only 20 MHz. In order to use the entire frequency band, it is necessary to attenuate at least 40 dB with a width of 1 MHz. It follows that a filter is required to be excellent in attenuation characteristics and to be low in an insertion loss. For obtaining such a filter, required is a resonator element having a high Q-value.
In addition, an individual amplifying system and a collective amplifying system are known to the art as an amplifying system in the transmitter section of a radio transmitter-receiver apparatus that synthesizes signals of at least two carrier frequencies for transmission.
The individual amplifying system is a system in which sets of signal generator, transmitter amplifier and transmitter filter are prepared in accordance with the number of carrier frequencies used, and the signals of the carrier frequencies outputted from each signal generator are individually amplified by the transmitter amplifiers and allowed to pass through the filters, followed by synthesizing signals in a power synthesizer so as to send the synthesized signals.
The collective amplifying system is a system in which signals of a plurality of carrier frequencies outputted from a plurality of signal generators are synthesized in a power synthesizer, followed by. collectively amplifying the synthesized signals in a single transmitter amplifier and subsequently allowing the amplified signals to pass through a filter and, then, sending the filtered signals.
The individual amplifying system is expected in principle to produce various advantages described below over the collective amplifying system.
Specifically, in the collective amplifying system, signals of a plurality of carrier frequencies simultaneously enter a single amplifier, with the result that mutual interference is brought about among the signals of each carrier frequency. What should be noted is that the power of a distorted signal caused by the mutual interference is likely to give adverse influences to the signal of another carrier frequency. In the individual amplifying system, however, only the signal of one carrier frequency enters a single amplifier, with the result that mutual interference is not brought about among the signals of each carrier frequency. Also, in the individual amplifying system, it is possible to prevent interference caused by turning of the signal of another carrier frequency by setting the pass band of the transmitter filter in the bands corresponding to the separate carrier frequencies. As a result, it is possible to power synthesize easily the signals of each channel in a synthesizer.
In general, in an amplifier, modulation distortion is generated when a modulating signal is amplified, with the result that it is possible for power to leak into the adjacent channel so as to bring about interference with the signal of that channel. Such being the situation, the upper limit of the leaking power into the adjacent channel is determined in the specification. For example, in the modulation system in which signal is contained in the amplitude component like QPSK, the modulation system is backed off and operated at a low efficiency so as to ensure a linearity of the amplifier. In this respect, only the signal of one carrier frequency is allowed to pass through the amplifier in the individual amplifying system so as to suppress the leaking power into the adjacent channel caused by the modulation distortion. It follows that it is possible to operate the amplifier at a high efficiency. Since the power consumption of the amplifier occupies a very large proportion in the entire radio transmitting apparatus, the improvement in the efficiency of the amplifier greatly contributes to the power saving of the radio transmitting apparatus.
In the collective amplifying system, it is possible to achieve about 40% of the maximum efficiency when the allowable maximum channels are contained. However, even where the number of channels used is small, required is power substantially equal to that in the case of using all the channels, leading to low power efficiency. In the individual amplifying system, however, it is possible to turn off the power supplies of the amplifiers for the channels that are not used so as to make effective that channels alone which are being used. It follows that it is possible to achieve the power saving.
It should also be noted that, in the collective amplifying system, the heat generation is concentrated on the amplifier so as to make it necessary to take a large-scale measure for the heat dissipation. In the individual amplifying system, however, a plurality of amplifiers forming heat sources are dispersed, making it unnecessary to take a large scale measure for the heat dissipation.
In order to make the synthesizer simple in construction in the individual amplifying system, it is necessary to use a filter satisfactory in selectivity. It should be noted this connection that it is difficult for the filter of the conventional waveguide type (dielectric cavity resonator type) to meet the required selectivity. On the other hand, the linearity of the amplifier is important in the collective amplifying system in order to avoid the mutual interference among the signals. In recent years, the linearity of the amplifier has been improved by various technical improvements. As a result, the collective amplifying system is used nowadays. However, it is desirable to use the individual amplifying system that has various advantages in principle as described above.
Under the circumstances, proposed in, for example, Japanese Patent Disclosure (Kokai) No. 2000-68958 is the idea of using the individual amplifying system, in which is used a filter comprising resonator elements having a high Q value formed thereon by using a superconductor so as to achieve a sharp cut.
It is conceivable to use a bulk and a thin film for utilizing a superconductor as the conductors of the filter, and it is convenient to use a thin film in view of the cooling method and the freedom of design. In particular, it is well known to the art to form a thin film on a substrate material of a very low loss such as sapphire or MgO and to process the thin film into a planar transmission line. A microstrip line structure, a strip line structure and a coplanar structure are used in many cases as the structure of the planar transmission line. These structures are compact and, thus, are advantageous over the filter structure of the conventional waveguide type (dielectric cavity resonator type).
However, the planar transmission line is exposed to the air in the free space, with the result that the transmitter signal is radiated into the free space so as to possibly give rise to the phenomenon that the electromagnetic field tends to leak from the transmission line. Under the circumstances, where a plurality of filters are arranged adjacent to each other, a serious problem is generated that the undesired radiation and the electromagnetic field leaking from the transmission line of one filter are allowed to interfere with the other filter, resulting in failure to obtain a sufficient SN ratio.
In particularly, in the base station of, for example, a cellular phone, both the transmitter signal and the receiver signal are handled and the transmitting and receiving circuits are arranged very close to each other. What should be noted in this connection is that the intensity of the signal transmitted from the base station is several orders higher than that of the signal received by the base station. It follows that, if the transmitter signal is mixed in the receiving circuit even if only slightly, it is impossible to process normally the received signals. In general, it is necessary to suppress the noise intensity relative to the original received signal intensity at 60 dB (one millionth) or less both inside and outside the receiver signal band. Originally, the undesired frequency (noise) is cut by the receiver filter. However, where a noise is mixed in the receiver filter itself or the transmission line behind, it is impossible to obtain a sufficient SN ratio, making it impossible to process the received signal.
FIG. 1 is a block diagram showing transmitter-receiver filter sections in a base station of, for example, a cellular phone. As shown in the drawing, a transmitter signal of a large power, which is generated from the signal generator 101 and passes through the power amplifier (PA) 102, is transmitted through a transmitter filter 1 and then, sent from the antenna 103. On the other hand, a weak receiver signal incident on the antenna 103 passes through the receiver filter 2. It should be noted that only the receiver signal frequency alone is selectively allowed to pass through the receiver filter 2 and the signal passing through the receiver filter 2 is amplified by the low-noise amplifier (LNA) 104 so as to be transmitted to the latter stage signal processing circuit 105. Since the signal intensity between the receiver filter 2 and the LNA 104 is weak, it is necessary to prevent the mixing of an undesired signal as much as possible. To be more specific, the SN ratio in the receiver filter is required to be at least 60 dB, as described previously.
In the case of employing the filter structure of the conventional waveguide type (dielectric cavity resonator type), the power does not leak from the transmitting circuit because the propagating portions of the microwave signal is covered with an outer wall, with the result that it is substantially unnecessary to worry about the mixing of noise into the receiving circuit. However, the cavity resonator type is a three-dimensional circuit, with the result that the freedom of design is limited and the circuit is rendered bulky. It follows that the filter structure of the cavity resonator type is unsuitable for the structure of a superconductor filter requiring cooling. It should also be noted that it is necessary to cover the entire inner surface of the cavity structure with a superconductor, leading to the problem that the manufacturing cost is increased.
On the other hand, the technology described below is known to the art as the means for alleviating the noise in the filter structure of the planar transmission circuit type.
For example, reported in Japanese Patent Disclosure No. 7-202507 is the structure that a single superconductor filter is housed in a brass case and the inner surface is covered with a radio wave absorber. In the case of this structure, however, it is necessary to prepare the superconductor filters one by one independently and to cover completely the superconductor filter with a brass case and a radio wave absorber. As a result, the filter structure is rendered bulky so as to sacrifice the compactness that is the feature of the planar transmission line. An additional problem is that the number of members that must be cooled is increased so as to increase the heat capacity, with the result that a long time is required for the cooling.
Incidentally, as seen in the recent packaging technology in the personal computer, vigorous studies are being made on an efficient layout that permits a large number of parts not to interfere with each other within a limited volume. However, the transmitting frequency of the signals is only several hundred MHz, or scores of centimeters to several meters in terms of the wavelength, in the personal computer. In other words, the size of each element is sufficiently smaller than the wavelength of the transmitting signal and, thus, the study noted above is directed to the discussion of the layout of the elements formed of a so-called lumped parameter circuit.
On the other hand, where the frequency of the transmitting signal is on the order of GHz like the superconductor filter, the wavelength is not longer than scores of centimeters (or the effective wavelength is not longer than several centimeters in view of the dielectric constant of the substrate constituting the transmission line) and, thus, required is the discussion on the layout of the elements in a so-called distributed parameter circuit. Under the circumstances, it is desirable to establish the packaging technology differing from the packaging technology on the personal computer.
Also, in the radio transmitter-receiver apparatus using a superconductor filter, it is desirable to utilize effectively the transmitting frequency band.
An object of the present invention is to provide a superconductor filter, which is free from interference even if a plurality of superconductor filters of the planar transmission line structure are arranged close to each other so as to obtain a sufficient SN ratio without sacrificing the compactness that is a feature of the planar transmission line structure, and which is excellent in the cooling efficiency.
Another object of the present invention is to provide a radio transmitter-receiver apparatus, which permits obtaining a good received state and is capable of effectively utilizing the transmitting frequency band.
According to one aspect of the present invention, there is provided a superconductor filter, comprising: a superconductor receiver filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line and configured to select a signal received from an antenna; a superconductor transmitter filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line and configured to select a signal transmitted to the antenna, a direction of the transmitter filter being arranged non-parallel to a direction of the receiver filter; and a heat-insulating container housing the superconductor receiver filter and the superconductor transmitter filter.
According to another aspect of the present invention, there is provided a superconductor filter, comprising: a superconductor receiver filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line and configured to select a signal received from an antenna; a superconductor transmitter filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line, and configured to select a signal transmitted to the antenna, the transmitter filter being arranged in a position out of alignment with the receiver filter along a signal transmitting direction; and a heat-insulating container housing the superconductor receiver filter and the superconductor transmitter filter.
According to another aspect of the present invention, there is provided a superconductor filter, comprising: a polyhedral cooling member; a superconductor receiver filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line, and configured to select a signal received from an antenna, the receiver filter being mounted on one surface of the cooling member; a superconductor transmitter filter of a planar transmission line structure including a signal input line, a resonator element and a signal output line, and configured to select a signal transmitted to the antenna, the transmitter filter being mounted on another surface of the cooling member differing from the surface on which the superconductor receiver filter is mounted; and a heat-insulating container housing the cooling member, the superconductor receiver filter and the superconductor transmitter filter.
Further, according to still another aspect of the present invention, there is provided a radio transmitter-receiver apparatus configured to perform communication by using at least two carrier frequencies, comprising: a plurality of radio transmitter-receiver units each including at least one transmitter unit and at least one receiver unit connected in parallel to a single antenna, the transmitter unit including a signal generator generating a signal of one carrier frequency used for communication, an amplifier amplifying the signal of the carrier frequency and a superconductor transmitter filter filtering a signal of a predetermined band from the amplified signal, which are connected in cascade connection, and the receiver unit including a superconductor receiver filter filtering a signal of a predetermined band from a signal of a single carrier frequency received by the antenna and an amplifier amplifying the signal of the predetermined band, which are connected in cascade connection; and a single receiver signal processing circuit to which the receiver units included in the plurality of radio transmitter-receiver units are connected in parallel.